Mississippi plume could threaten life in Gulf

More than a trillion gallons of polluted water — a volume equal to Tampa Bay — cascaded from the flood-swollen Mississippi Delta watershed into the Gulf of Mexico daily during May. Now, scientists say, the vast plume could trigger toxic algae blooms and harm sea life as far away as Southwest Florida and the Florida Keys.

Some of that dirty water is circulating in a large 10,800-square-mile eddy about 150 miles west of Sarasota. Another smaller, but more concentrated slug is flowing southeast toward the Florida Keys.

Loaded with nutrients, pesticides and other land-based pollutants, the contaminated water may feed blooms of toxic algae or create marine-life-killing "dead zones."

They also could douse coral reefs with toxins or drive fish from spawning grounds off the Southwest Florida coast.

"We have never seen a pulse of this type of water coming in," said Mitchell Roffer, a biological oceanographer who runs a prominent fishing forecast service. "This is probably the largest pulse of fresh, discolored water to ever reach the keys."

Growing signs of a troubled Gulf ecosystem have emerged without a clear explanation or obvious links to the BP Deepwater Horizon oil spill last year or the Mississippi flooding this year.

Hundreds of dolphins and sea turtles have washed ashore dead in Louisiana, Mississippi and Alabama. Lesions have been found on fish in the northern gulf and a large fish kill recently littered beaches in Collier County with hundreds of rotting fish.

Storm-water runoff from 40 percent of the U.S. flows into the Mississippi, including from the Midwest's vast agriculture belt and major cities, including Chicago, St. Louis and Memphis, Tenn. During record floods this year the river jumped its banks, sweeping away homes and cars and scooping up sediments as it scoured the land.

River flows to the Gulf in May were the highest recorded since 1973 and the rate of discharge to the Gulf exceeded records going back to 1930, according to a report in June.

That report predicted an unprecedented dead zone forming in the northern Gulf as a result. The dead zone is caused by a proliferation of algae and bacteria that rob the sea of oxygen, suffocating the creatures trapped in it.

But the algae-spawning Mississippi plume is spreading to other areas of the Gulf of Mexico.

If it reaches the Florida Keys and lingers it could damage coral reefs and fish that inhabit them. It also could stir up toxic material left over from the BP Deepwater Horizon disaster, said Jerry Ault, a biologist with the University of Miami's Rosenstiel School of Marine and Atmospheric Science.

Ault and a team of other scientists, including physical oceanographers, are studying the plume's movement and impact. He said he is concerned that a combination of toxins from the oil spill and the Mississippi plume could hinder spawning by tarpon and other valuable sport and commercial fish off Florida's west coast.

"This balance of the ecosystem, if you will, is affected by these far afield inputs," Ault said.

The BP spill and the Mississippi floodwater have compounded chronic strains on the Gulf from oil and gas extraction, overfishing and storm-water runoff.

"The Gulf is under significant pressure," Ault said.

The dead zone's size depends on the amount of nutrients in the Mississippi and the weather. A stormy season mixes oxygen into the water. More calm weather this year could contribute to a record-breaking dead zone covering an area the size of Delaware and New Jersey combined.

Mississippi water usually flows west, muddying the northern Texas beaches. The difference this year is the Mississippi's volume, impacting a wider area of the Gulf.

Past studies on much smaller Mississippi discharges have linked red tides in Southwest Florida to nutrient-rich water moving from the Delta to the edge of the west Florida shelf.

Red tide blooms have been conspicuously absent from Southwest Florida since the winter of 2007.

Scientists think the fish kill in Collier was caused by oxygen depletion from an algae bloom, but the bloom's origin is unclear. Ault said the problem could have stemmed from excessive local storm-water runoff.

Even if the Mississippi water causes little discernible negative effects for Southwest Florida or the Keys, large species of fish that swim in Southwest Florida waters rely on the northern Gulf for a seasonal feast of menhaden — a small baitfish that usually flourishes in riverine plume waters.

"That area where the Mississippi comes out is super important as a prey source," Ault said, suggesting that recent blows from oil and pollution could lead to major fish kills or chronic reproductive problems that appear much later. "It's this combination of effects. It's not a single source. We're adding to a stressed situation."